Tennessee Wastewater Operator Practice Exam

What effect does an increased flow rate to primary clarifiers have on detention time and surface loading rate?

1. Detention time increases; loading rate decreases

2. Detention time decreases; loading rate increases

3. No significant effect

4. BOD removal efficiency improves

The correct answer is: Detention time decreases; loading rate increases

An increased flow rate to primary clarifiers leads to a decrease in detention time because the water passes through the clarifier more quickly. Detention time refers to the time that wastewater is held in the clarifier for settling to occur. When flow increases, the volume of water coming in reduces the time that any given volume of that water stays in the clarifier. Simultaneously, the surface loading rate increases due to a higher volume of water entering the clarifier area per unit time. The surface loading rate is calculated by dividing the flow rate by the surface area of the clarifier. With increased inflow, and assuming the surface area remains constant, this rate rises, meaning that more water is being processed in the same space over a shorter time, which can affect settling efficiency. Understanding this relationship is critical in wastewater treatment operations since it impacts the overall effectiveness of solid separation in the primary clarifier, influencing downstream processes such as biological treatment and the removal of biochemical oxygen demand (BOD). Effective management of detention time and loading rates is essential for optimal system performance.